Endothelial cells (EC) effectively present antigen to resting peripheral blood lymphocytes in vitro, and many initiate diverse immune reactions in vivo, including delayed type hypersensitivity, vasculitis and graft arteriosclerosis. Indeed, EC are antigen presenting cells (APC) uniquely positioned to recruit circulating T cells at sites of antigenic challenge and provide the initial signal secretion of interleukin-2 (IL-2), the major autocrine and paracrine T cell growth factor. Endothelial costimulatory signals are delivered through CD2, along with unidentified ligands on the T cell, that result in increased IL-2 gene transcription and resistance of IL-2 synthesis to immunosuppression by cyclosporin A (CsA). These costimulatory signals underlie the ability of EC to activate resting T cells. The major hypothesis of this proposal is that EC provide a unique combination of costimulatory signals that activate combinations of transcription factors distinct from those induced by other APC, such as monocytes, B cells or dendritic cells. These differences may account for the CsA resistant nature of EC costimulation. It is the overall goal of this proposal to identify the surface ligands involved in EC costimulation and to characterize the T cell nuclear factors induced by EC that are involved in IL-2 transcription. Specifically the aims are; (1) to identify T cell transcription factors that are induced by EC costimulation and bind to the IL-2 promoter, using mobility-shift assays and analysis of mRNA; (2) to identify, by using antisense oligonucleotide inhibition of translation and in vitro transcription assays, those nuclear factor that, by binding to the promoter, induce IL-2 transcription; (3) to identify, by monoclonal antibody blocking, molecules on the surface of resting and activated EC involved in activating specific transcription factors in T cells; and (4) to compare the pathways and costimulatory molecules used by large vessel EC, microvascular EC and other APC such as B cells and dendritic cell. The information gained from these experiments will provide a basis for selectively modulating immune responses triggered through antigen presentation by EC versus other APC, and may guide strategies aimed at preventing unwanted immune activation by EC.